In the manufacture of circuit boards, especially for use in computers, various components are mounted on the board. Among these components are included integrated circuit (I/C) chips. Different techniques have been developed for mounting these I/C chips onto circuit boards. One of these techniques is a so-called direct chip attach (DCA), in which the chips are mounted along with other components directly onto the circuit board or mother board of the computer. This type of mounting does have certain attractive aspects; however, there are some serious limitations to direct chip attach. These limitations include the possibility of thermal mismatch of the silicon chips with the circuit board, which is typically formed of epoxy fiberglass (FR4 technology) which has a significantly different coefficient of thermal expansion (CTE) from that of the integrated circuit chip which may be formed of silicon. Moreover, if re-working of the circuit board is required because of defective chips, this is oftentimes very difficult to do and can, in extreme cases, result in totally scrapping the circuit board at the end of the manufacturing process.
In order to avoid the above difficulties, as well as others, one common technique for attaching I/C chips to circuit boards is by utilization of chip carriers. These chip carriers mount the I/C chips, which carriers are then in turn mounted to the circuit board. While this does introduce another level of packaging, nevertheless there are certain advantages to this type of chip mounting which in certain instances make it a more desirable mode of attaching a chip to the circuit board. With this technique, the integrated circuit chip is secured to a chip carrier either by solder ball connections or wire bonding. The chip carrier, with one or more chips attached thereto, is attached to the circuit board by various different technologies such as pin-in-hole, solder ball or other techniques. This utilization of a chip carrier has the advantage of allowing each of the chips to be tested individually or in small groups on the chip carriers before attaching to the board and indeed after attaching to the board, and if there is a defective or malfunctioning chip, that chip can be easily replaced. If it is not possible to replace the chip, then the chip carrier can be discarded without discarding or scrapping the entire circuit board at a late stage in the processing. This technique also allows for greater tolerances to thermal mismatch between the chip and the chip carrier for several reasons.
In one technique which utilizes a chip carrier, the carrier material can be selected which has a coefficient of thermal expansion between that of the integrated circuit chip and the organic material of the circuit board. One such type of carrier is a ceramic carrier which is well-known in the art.
Another factor in eliminating or minimizing the consequences of thermal mismatch between the chip and the circuit board material is that the chip carrier itself is relatively small as compared to the circuit board. Thus, the effect of thermal mismatch, vis-a-vis, the chip, is significantly reduced as compared to situations where there is direct chip attach with relatively small integrated circuit chips and a relatively large circuit board. Hence, a technique for mounting chips on carriers and carriers to circuit boards has been developed in which the same material is used for both the chip carrier and the circuit board, which in many instances is FR4 epoxy fiberglass material. By utilizing the same material for both the chip carrier and the circuit board, essentially identical technologies can be used to manufacture both the chip carriers and the circuit board, thereby reducing the number of technologies necessary to provide a finished product.
Technologies used to form epoxy fiberglass FR4 chip carriers include, among other things, lamination of several layers of the FR4 organic board together to form the chip carrier, and also include photolithographic techniques where photoresist, either positive-acting or negative-acting, is used to pattern the material for metal deposition. Because of their configurations, both of these processes can have certain detrimental effects on the manufacturing process which can lead to causing significant problems with the resultant chip carrier to the extent that it is unsatisfactory for the intended use. Thus, it is desirable to eliminate these problems in the FR4 technology.
In the above-noted parent application, Ser. No. 08/359,491, a technique for forming chip carriers having openings therein in which chips are mounted is described in which multiple layers with pre-formed openings therein are utilized. A plug is provided which fits in the openings and photoresist bridges over the opposite sides of the plug to protect the I/C chip area from damage during circuitization and other wet processing of the chip. While this technique works quite well in many instances, nevertheless it does require pre-drilling or other forming of the board components with aligned openings and requires the use of a separate piece which must be removed after processing and relies on the seal of photoresist to prevent unwanted contamination during wet processing. In some instances, this technique is not only more expensive, but has certain limitations on how it can be used.